1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens suitable for a photographic optical system used in an image pickup device such as a video camera, a surveillance camera, a digital still camera, a broadcasting camera, and a silver-halide photographic camera.
2. Description of the Related Art
A photographic optical system used in the image pickup device is required to be a compact zoom lens having a wide angle of view, small aberrations a high zoom ratio, etc. An exemplary zoom lens that includes in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power has been known.
Japanese Patent Application. Laid-Open No, H08-82743 and U.S. Pat. No. 8,284,498 disclose a rear-focus-type four-unit zoom lens configured to change magnification by moving second and third lens units and correct image plane variations associated with the variable magnification by moving a fourth lens unit, while carrying out a focusing operation. U.S. Pat. No. 7,477,457 discloses a zoom lens configured to carry out a zooming operation by moving the four lens units, and a focusing operation is carried out by the fourth lens unit.
In the above-described positive-lead-type four-unit zoom lens, it is relatively easy to reduce the size of the entire zoom lens and obtain a high zoom ratio. To achieve a high optical performance over the entire zooming range while attempting to obtain a wide angle of view and a high zoom ratio, it becomes important to individually set the zoom type, refractive power configurations, etc., of the units appropriately. For example, in the rear-focus-type four-unit zoom lens of the above-described zoom type, it would be important to appropriately set the refractive power, a moving interval (moving amount) associated with zooming operation, an image forming magnification both at a wide angle end and a telephoto end, etc., of the second lens unit used for variable magnification.
Further, it becomes important to appropriately set the refractive power, image forming magnification at a wide angle end and a telephoto end, etc., of the third lens unit. Without setting these configurations appropriately, it would be difficult to achieve a high optical performance over the entire variable magnification range and to obtain a wide angle of view and high zoom ratio while reducing the size of the entire zoom lens.